Previously, we have identified and purified a 69kDa surface protein (pertactin) found on virulent strains of B pertussis. We found that this protein protects mice against infection following a respiratory challenge with B. pertussis. Three hybridomas were established that produce monoclonal antibodies (Mabs) directed against this protein. One of these Mabs was found to passively protect mice in the aerosol disease model. The epitopes for the Mabs were identified using fusion proteins of various lengths constructed from the P. 69 gene. An immunodominant and "protective" epitope was found near the C-terminus of the protein.